Alternating pressure pads are known and have been used in the construction of mattresses, chairs, couches and other patient support devices to improve blood circulation, and thereby reduce the incidence of bed sores.
U.S. Pat. No. 5,396,671 discloses an alternating pressure pad particularly designed for use as a mattress, in which two sets of inflatable cells are alternately inflated and deflated in order to stimulate blood circulation in a patient lying on the mattress. In this U.S. patent, a sensor pad is located beneath the inflatable cells in order to detect when the cells are insufficiently inflated. One described sensor comprises a flexible tube positioned beneath the mattress. One end of this tube is connected to the inflatable cells and the other discharges to atmosphere, thus providing a continuous leakage path. In the event that inflation pressure in the cells becomes insufficient to maintain the user above a base surface, the sensor tube will be compressed, reducing or entirely cutting off the flow of air therethrough. As a result, more air will become available to the cells and the user will be raised off the base surface, thereby releasing the compression of the tube and restoring the controlled leak through the sensor tube. While simple in operation, a sensor tube of this kind has the disadvantage that there is a constant loss of air from the system which necessitates the continuous operation of a higher capacity pump than would be required if such a leak could be avoided.